A thorough test of a high-speed digital system typically includes a procedure for sampling signals from hardware components of the system (e.g., circuit boards, interconnects, silicon devices, etc.) to determine how these components affect signal integrity. To access these signals, an engineer or technician typically connects specialized measurement equipment to the system hardware. In some configurations, such equipment includes a measuring device (e.g., a Time Domain Reflectometer or TDR) that connects to a circuit board which is either (i) part of the high-speed digital system under test, or (ii) a specialized assembly (e.g., a daughter card) that closely integrates with the system under test (e.g., through high-density connectors). Typically, the engineer solders a coaxial radio frequency (RF) connector to a specialized feature of the circuit board called a signal launch. The engineer can then attach a coaxial cable (e.g., a 50 ohm cable) from the measuring device to the soldered RF connector in order to access signals of the system under test.
One type of signal launch, which is hereinafter referred to as an edge mount signal launch, resides along an edge of a special type of circuit board called a microstrip, or microstrip line, which has an exposed signal conductor on one side of a dielectric substrate and an exposed ground conductor on the other side. Both the signal and ground conductors typically run to the edge of the circuit board. An engineer typically attaches an edge mount coaxial RF connector having outer soldering posts and an inner soldering post to the circuit board. In particular, the engineer solders the inner soldering post of the connector to the signal conductor and the outer soldering posts to the ground conductor. The engineer can then access signals of the system under test by attaching a coaxial cable from a measuring device to the connector.
Another type of signal launch, which is hereinafter referred to as a surface mount signal launch, resides on the surface of a circuit board rather that along an edge in order to receive a connector that mounts perpendicularly relative to the circuit board surface. A circuit board having multiple surface mount signal launches may position these signal launches in a circular fashion around a component mounting location (e.g., an array of vias for a high-density connector) such that etch lengths from the mounting location to the signal launches are of equal lengths for signal matching purposes. A conventional surface mount signal launch typically includes a large centrally-located signal via (i.e., a plated-through hole) and four large ground vias that are equidistant from the signal via. An engineer typically places a surface mount coaxial RF connector having a thick centrally located signal post and four thick surrounding ground posts over the surface mount signal launch such that each post inserts into a respective via (the signal post in to the signal via and the ground posts into the ground vias). The engineer then solders the posts to the surface mount signal launch. The engineer can then attach a coaxial cable from the connector to the measuring device in order to access signals of the system under test using the measuring device.
Unfortunately, there are deficiencies to the above-described conventional types of signal launches. For example, in connection with the conventional edge mount signal launches, it is difficult to position multiple edge mount signal launches in a circular fashion around a component mounting location (e.g., an array of vias for a high-density connector) such that etch lengths from the mounting location to the signal launches are equal for signal matching purposes. Circuit boards are typically rectangular in shape (i.e., have straight edges rather than circular edges) thus making it difficult to position edge mount signal launches in a circular manner. Additionally, a typical circuit board has a relatively small amount of perimeter length (vis-à-vis surface area) thus limiting the number of edge mount signal launches that can be conveniently placed on that circuit board when compared to the number of surface mount signal launches that can be placed on a circuit board. Furthermore, edge mount signal launch connections are highly susceptible to damage and fatigue over time due to there location along the circuit board edge. For example, a relatively a small amount of torque on an edge mount connector can stress the connector, solder joints and circuit board in that area to a point that causes a fracture in the solder joint or perhaps physical damage to the connector and/or the circuit board.
In connection with conventional surface mount signal launches, such launches are typically not well impedance-matched with their coaxial surface mount RF connector counterparts, and are thus a source of signal distortion. In particular, the centrally-located signal via and surrounding ground vias are typically made large for improved manufacturability (simpler drilling and plating) and soldering (more room to accommodate the thick connector posts and promote solder flow during the soldering process). Unfortunately, the large size of the centrally-located signal via reduces the amount of clearance between the signal via and surrounding ground conductor within the circuit board (i.e., ground planes) thus providing a high capacitance effect between the signal via and the ground conductor. Additionally, the large size of the signal via forces the ground vias to be placed substantially away from the center of the signal launch, the distance being uncharacteristically greater than the distance between the inner conductor and the outer conductor of standard coaxial cable (e.g., 50 ohm cable) thus creating inductance loops within the surface mount signal launches. The high capacitance effect and created inductance loops tend to distort signals through the surface mount signal launches thus degrading signal integrity.
Unfortunately, for the conventional surface mount signal launch, it would be difficult to counteract the above-described transmission line effects by changing the size of the signal via or positions of the ground vias. In particular, it would be difficult to decrease the size of the signal via because the conventional surface mount signal launch must accommodate soldering of a thick signal post of the coaxial RF connector. Furthermore, it would be difficult to move the ground vias closer to the signal via since the conventional surface mount signal launch solders to the RF connector, and an adequate amount of clearance is required between the ground vias and the signal via to prevent mechanical mounting problems and soldering problems.
In contrast to the above-described conventional signal launches, the invention is directed to techniques for forming a connection between a circuit board and a connector using a signal launch having multiple sets of ground vias. One set of ground vias enables the connector to mount securely to the circuit board (e.g., using solder, screws and nuts, etc.), while another set of ground vias provides electrical pathways that more closely match conductor clearances of a coaxial cable which can connect the connector to a measuring device (e.g., a TDR). Such pathways decrease signal distortion through the signal launch and the connector (e.g., keep any inductance loops small) thus improving the accuracy of signal measurements from the circuit board.
One arrangement of the invention is directed to a connection system having a circuit board that includes (i) a section of circuit board material having a signal conductor, a ground conductor, and dielectric material that physically separates the signal conductor and the ground conductor, and (ii) a signal launch. The signal launch includes a signal via that physically contacts the signal conductor and the dielectric material of the section of circuit board material. The signal launch further includes a first set of ground vias and a second set of ground vias, each of which physically contacts the ground conductor and the dielectric material. Each of the first set of ground vias is disposed a first radial distance from the signal via (e.g., 0.100 of an inch or 100 xe2x80x9cmilsxe2x80x9d from the center of the signal via). Each of the second set of ground vias is disposed a second radial distance from the signal via (e.g., more than 140 mils from the center of the signal via). The connection system further includes a coaxial connector that mounts to the signal launch of the circuit board in order to provide electrical access to the signal and ground conductors of the circuit board. The different sets of ground vias provides flexibility enabling one set to operate as a mounting holes (e.g., for soldering or bolting the connector to the signal launch), and the other set to provide improved transmission line characteristics (smaller inductance loops, etc.) in order to reduce signal distortion.
In one arrangement, the signal launch further includes a ground pad disposed on a surface of the section of circuit board material. The ground pad physically contacts each of the first and second sets of ground vias of the signal launch and the dielectric material of the section of circuit board material. The ground pad enables improved electrical contact between the connector and the signal launch by providing additional surface area that contacts the connector. In particular, portions of the ground pad that are close the signal via can contact conductive portions of the connector that are close to the signal conductor of the connector in order to minimize inductance loops. Additionally, the ground pad ties each of the ground vias together for improved signal distribution.
In one arrangement, the signal launch includes a ground pad on both sides of the circuit board. This arrangement provides symmetry to accommodate the standard practice of many circuit board manufacturers of making each side of the circuit board symmetrical.
In one arrangement, the first radial distance is smaller than the second radial distance such that the first set of ground vias is disposed closer to the signal via than the second set of ground vias. Furthermore, the signal via has an inner diameter (e.g., 15 mils) that is smaller than an inner diameter of each of the first set of ground vias (e.g., 22 mils). In this arrangement, the signal via has a narrow inner diameter that is convenient for receiving a pin but allows the first set of the ground vias (i.e., the closest set) to have a somewhat larger inner diameter for improved manufacturability (e.g., simpler drilling and plating) and signal integrity purposes. The smaller size of the signal via allows for a larger clearance or void between the signal via and ground planes within the circuit board (i.e., a larger anti-pad) over that of conventional surface mount signal launches thus improving the transmission line characteristics (e.g., the capacitance effect) of this arrangement for less signal distortion.
In one arrangement, the first radial distance is smaller than the second radial distance such that the first set of ground vias is disposed closer to the signal via than the second set of ground vias. Additionally, the signal via has an inner diameter (e.g., 15 mils) that is smaller than an inner diameter of each of the second set of ground vias (e.g., 67 mils). In this arrangement, the signal via has a narrow inner diameter that is convenient for receiving a pin but allows the second set of ground vias (the ground vias farthest from the signal via) to have a large inner diameter that enables mounting of the connector to the circuit board (e.g., bolting, soldering, etc.).
In one arrangement, the first radial distance is smaller than the second radial distance such that the first set of ground vias is disposed closer to the signal via than the second set of ground vias. Furthermore, each of the first set of ground vias has an inner diameter (e.g., 22 mils) that is smaller than an inner diameter of each of the second set of ground vias (e.g., 67 mils). In this arrangement, each of the first set of ground vias has a smaller inner diameter enabling it to be positioned closer to the signal via for improved signal integrity purposes, i.e., to better imitate the conductor clearance of a coaxial cable yet still maintain a large anti-pad. In one arrangement, the dielectric material of the section of circuit board material separates the first set of ground vias from the signal via by less than 82 mils. In this arrangement, each of the second set of ground vias (the outer set) has a larger inner diameter enabling it to fasten the connector to the circuit board (e.g., by soldering, bolting, etc.).
In one arrangement, the signal launch further includes a signal pin that electrically connects with the signal conductor of the section of circuit board material through the signal via. In particular, the signal pin extends perpendicularly from a plane of the section of circuit board material. This arrangement enables a female section of a connector to at least temporarily mount (e.g., engage) with the signal launch thus providing the option of removing the connector at a later time (e.g., to replace the connector, to reuse the connector on a new signal launch, etc.). Furthermore, the pin preferably has a narrow diameter (e.g., 11 mils) to reduce the capacitance effect between signal and ground conductors.
In one arrangement, the signal pin has a diameter (e.g., 11 mils) that is less than an inner diameter of the signal via (e.g., 15 mils), and the signal pin connects to the signal via through a solder joint. In this arrangement, the solder joint provides a reliable and robust electrical connection between the pin and the signal via.
In an alternative arrangement, at least a portion of the signal pin has a diameter (e.g., 15 mils) that is greater than or equal to an inner diameter of the signal via (e.g., 15 mils), and the signal pin connects to the signal via in a press-fit manner. In this arrangement, the pin is removable from the signal via thus improving the flexibility of the connection system (e.g., removing the pin in order to later mount a male connector over the signal launch).
In one arrangement, each of the first set of ground vias is disposed between the signal via and a respective one of the second set of ground vias. The placement of the first set of ground vias in line with the second set of ground vias thus can assist in minimizing, counteracting, or shielding the signal via from, any negative inductance loop affects provided by the second set of ground vias. Additionally, the placement of the first set of ground vias in line with the second set of ground vias enables a signal conductor to easily lead to the signal via within the circuit board without having to wind around any ground vias.
The features of the invention, as described above, may be employed in testing systems, devices and methods and other computer-related assemblies such as those of Teradyne Corporation of Boston, Mass.